#!/usr/bin/perl

use strict;
use File::ReadFileBackwards;

my @instances = ();

$file=File::ReadFileBackwards->new("/some/log/file");
while (@instances < 10 and defined($line = $file->readline))
{
  push @instances, $line;
}

##</code><code>##

#!/usr/bin/perl

use strict;
use File::Tail;

my @instances = ();

$file=File::Tail->new("/some/log/file");
while (defined($line=$file->read)) {
  if ($line =~ m/criteria/) {
    my $discard = pop @instances if @instances > 10;
    push @instances, $line;
  }
}